1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle, and more particularly to an anti-skid control system for controlling a braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability or the controllability is influenced detrimentally depending upon the road surface condition, when road wheels are locked in abrupt braking operation. In view of the fact that, when a hydraulic braking pressure supplied to each wheel brake cylinder is increased, the rotational speed of the road wheel is rapidly reduced immediately before the coefficient of friction relative to the road wheel reaches a maximum, the wheel cylinder pressure is controlled in accordance with the deceleration of the vehicle in order that a slip rate of the road wheel results in around 20%, that is, the maximum coefficient of friction is obtained.
In order to prevent the road wheels from being locked in abrupt braking operation, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding the hydraulic braking pressure supplied to wheel brake cylinders.
According to the anti-skid control system, in general, detected is a rotational speed of each road wheel or wheel speed, on the basis of which an estimated vehicle speed is set as a standard speed in response to an output from an acceleration sensor. Based upon the estimated vehicle speed, another standard speed is calculated and compared with each wheel speed. In response to the result of this comparison, controlled is the hydraulic braking pressure in the wheel brake cylinder mounted on each road wheel.
For example, as shown in FIG.11, a standard speed Vsn is set to follow an estimated vehicle speed Vso with a certain speed difference. If a wheel speed Vw becomes lower than the standard speed Vsn (Vw&lt;Vsn), and a wheel acceleration (including deceleration as negative value) Gw becomes lower than a standard acceleration G1 (Gw&lt;G1) which are omitted in FIG.11, then a hydraulic braking pressure in the wheel brake cylinder is decreased. If it is detected that the wheel acceleration Gw becomes equal to or more than the standard acceleration G1 (Gw.gtoreq.G1) so that the wheel speed Vw is recovering its speed, the hydraulic braking pressure is held as it is, whereas if the wheel acceleration Gw becomes equal to or more than the standard acceleration G1 (Gw.gtoreq.G1) and the wheel speed Vw becomes equal to or more than the standard speed Vsn, the hydraulic braking pressure is increased.
The estimated vehicle speed Vso is set on the basis of the wheel speed Vw, an output of an acceleration detecting apparatus and the like, such that its decreasing rate will be greater than a decreasing rate of an actual vehicle speed V. Especially, in the case where the estimated vehicle speed Vso is set on the basis of the output of the acceleration detecting apparatus such as an acceleration sensor, an acceleration switch or the like, the estimated vehicle speed Vso is influenced largely by road conditions such as an inclined road surface, various noises caused by a vehicle vibration or the like. Therefore, the decreasing rate of the estimated vehicle speed Vso is set to a rate large enough to compensate for a difference between the actual deceleration and the detected deceleration. Consequently, the wheel speed Vw is controlled to decrease with a decreasing rate greater than that of the actual vehicle speed V. The difference between those speeds will be large, especially in the case where the vehicle is running on a road surface whose coefficient of friction (hereinafter, simply referred to as CF) is low.
As described above, if the anti-skid control operation using the standard speed Vsn is performed on a road with a low-CF surface, or a low-CF road, the wheel speed Vw will decrease in accordance with the estimated vehicle speed Vso before recovering the speed sufficiently to thereby cause a rapid locking of the wheel. In order to avoid this, generally employed is a control by which a decreasing mode is changed over to an increasing mode at the time when the value of the wheel acceleration Gw changes from a positive value to a negative value, i.e., changing from an increasing wheel speed to a decreasing wheel speed.
That is, the hydraulic braking pressure is controlled to be increased when a high peak of the wheel speed Vw is caused as shown in FIG.12. According to this control, the rapid locking of the wheel on the low-CF road may be avoided, while the timing for increasing the hydraulic braking pressure will be delayed on a road with CF higher than that of the above road, such as a middle-CF road or a high-CF road, so that the stopping distance will be extended. In FIG.12, the hydraulic braking pressure characteristic indicates a delay in increasing the pressure is caused as shown by a solid line against a desired pressure increasing as shown by a phantom line. Consequently, the vehicle acceleration (deceleration in this case) is once reduced to cause not only a detrimental braking feeling, but also an extension of stopping distance with a range indicated by hatching in FIG.12. This extension of stopping distance caused by the delay in increasing the hydraulic braking pressure is large when the vehicle is running at high speed on the high-CF road, while it is small when the vehicle is running at low speed on the low-CF road.
In Japanese Patent Laid-open Publication No. Sho 63-195055, it is pointed out that the prior anti-locking control system starts increasing the hydraulic braking pressure at a high peak (a salient inflection point) of the wheel speed Vw, irrespective whether the coefficient of friction of the road surface (the road CF) is high or low, a timing for increasing the pressure is delayed on a middle-CF road or high-CF road to thereby overshoot the wheel speed Vw adjacent the high peak thereof, so that the stopping distance will be extended or a vehicle body will vibrate. To solve this problem, it is proposed in the above publication that a starting time for increasing the pressure shall be advanced on the middle-CF road or high-CF road. Namely, it is arranged that the starting time for increasing the pressure on the middle-CF road or high-CF road is changed to the time when the wheel speed decreases down to a low peak and then increases up to a predetermined speed Vt.
However, in the case where the decreasing rate of the estimated vehicle speed Vso is greater than that of the actual vehicle speed V as shown in FIG.11, if the starting time for increasing the pressure is set to the time when the wheel speed Vw reaches the predetermined speed Vt as proposed in the above publication, the wheel speed Vw decreases in accordance with the estimated vehicle speed Vso, so that the rapid locking of wheel may be caused. To avoid this, if the anti-skid control using the high peak of wheel speed as described above is employed, the stopping distance will be extended, especially in the case where the vehicle is running at high speed.